# Catalyst problem or just bad sensor?

**URL:** <https://community.cartalk.com/t/catalyst-problem-or-just-bad-sensor/168362>\
**Category:** Maintenance/Repairs\
**Tags:** impreza, subaru\
**Created:** [August 6, 2020, 3:54pm UTC](https://community.cartalk.com/t/catalyst-problem-or-just-bad-sensor/168362 "2020-08-06T15:54:43Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Robert-431](https://avatars.discourse-cdn.com/v4/letter/r/f04885/32.png) [@Robert-431](https://community.cartalk.com/u/Robert-431)\
**Post date:** [August 6, 2020, 3:54pm UTC](https://community.cartalk.com/t/catalyst-problem-or-just-bad-sensor/168362/1 "2020-08-06T15:54:43Z")

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My 2006 impress is showing a po420. My charge wants to replace the cat. I am not seeing or feeling any loss of performance. No visible emission. Is it reasonable it’s just the ox sensor? I don’t if the garage checked it.likely just plugged in reader. Also I might add that the light pretty much stays on but will go off from time to time then back on. Loose wire?

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**Author:** ![Tester](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/tester/32/15976_2.png) [@Tester](https://community.cartalk.com/u/Tester)\
**Post date:** [August 6, 2020, 3:58pm UTC](https://community.cartalk.com/t/catalyst-problem-or-just-bad-sensor/168362/2 "2020-08-06T15:58:48Z")

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Have your mechanic monitor the O2 sensors real-time.

It’ll confirm if the catalytic converter is bad.

[![](https://global.discourse-cdn.com/cartalk/original/3X/9/2/920c82aae79279b7169d496b337e65aa8eb81476.jpeg "Using upstream and downstream oxygen sensors to test the catalytic converter") ](https://www.youtube.com/watch?v=KGpMzdEO5KY)

Tester

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**Author:** ![texases](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/texases/32/862_2.png) [@texases](https://community.cartalk.com/u/texases)\
**Post date:** [August 6, 2020, 4:02pm UTC](https://community.cartalk.com/t/catalyst-problem-or-just-bad-sensor/168362/3 "2020-08-06T16:02:25Z")

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What @Tester said. If it is the cat, there won’t be any visible signs, either driving it or looking at the exhaust. Just have the mechanic make sure it’s not the oxygen sensor(s).